1. Field of the Invention
The present invention relates to a column electrode driving circuit for use with an image display device for displaying images, such as characters and/or (still or moving) pictures; and an image display device incorporating such a column electrode driving circuit.
2. Description of the Related Art
A liquid crystal display device, which is one type of image display device, includes a liquid crystal display panel, which is composed essentially of a liquid crystal layer interposed between a pair of glass substrates. On one of the glass substrates of the liquid crystal display panel, a plurality of data lines (column electrodes) which are disposed in parallel to one another, and a plurality of scanning lines (row electrodes) which perpendicularly intersect the respective data lines are provided. A voltage which is applied to each pixel of the liquid crystal display panel is controlled based on a voltage which is applied to each data line. The data lines are driven by a source driver IC, which functions as a column electrode driving circuit IC.
FIG. 3 is a block diagram illustrating the internal structure of a source driver IC 1, which functions as a column electrode driving circuit IC for a conventional color liquid crystal display panel. The illustrated source driver IC 1 (column electrode driving circuit IC) has 384 outputs. The source driver IC 1 includes a shift register 2, a sampling memory 3, a hold memory 4, a D/A converter 5, an output circuit 6, and a reference voltage generation circuit 7.
The shift register 2 receives a clock signal CK and a sampling start signal SP, which are transmitted from a signal control circuit (not shown), and outputs data sampling signals to the sampling memory 3.
In accordance with the timing of the data sampling signals which are output from the shift register 2, the sampling memory 3 latches a 6-bit data signal for each color of RGB (Red, Green, Blue), which is transmitted from the signal control circuit (not shown), and stores the 6-bit data signals as 6-bit sampling data. In the case where the source driver IC 1 (column electrode driving circuit IC) has 384 outputs, the sampling memory 3 has 128 outputs for each color of RGB (i.e., a total of 384 outputs). Each of the 384 outputs is stored as 6-bit sampling data.
The 6-bit sampling data stored in the sampling memory 3 are transferred based on a data transfer signal LS which is output from the signal control circuit (not shown). The hold memory 4 stores the transferred 6-bit sampling data.
Sixty-four reference voltage lines are coupled to the D/A converter 5 from the reference voltage generation circuit 7. Sixty-four levels of voltages (corresponding to 6 bits), which are output from the reference voltage generation circuit 7, are respectively supplied on the 64 reference voltage lines. A digital/analog conversion switch (not shown) is provided for each reference voltage line. The D/A converter 5 selects a 6-bit data signal for each color of RGB (i.e., the 6-bit sampling data stored in the hold memory 4) in accordance with a designated signal level, and converts the selected signal into an analog signal to be output. Specifically, the D/A converter 5 selects (by means of the digital/analog conversion switches) one of the reference voltage lines in accordance with a designated signal level for the 6-bit data signal for each color of RGB, and outputs a signal which has been converted into an analog signal to the output circuit 6.
The output circuit 6 subjects the analog signals which have been converted by the D/A converter 5 to impedance conversion, and output the resultant analog signals as driving voltages to the data lines coupled to the respective output nodes.
In the case of a liquid crystal display panel of a liquid crystal display device, employing a DC current to drive the liquid crystal material may allow electrolysis or the like to occur at the electrode surface, whereby a rapid deterioration of the liquid crystal display panel may occur. Therefore, the liquid crystal material is typically driven by an AC driving method, in which the polarity of a voltage applied to the electrodes of the liquid crystal display panel is alternated between positive and negative. In this case, however, each of the 64 reference voltage lines, to which one of the aforementioned reference voltages of 64 levels (corresponding to 6 bits) is to be applied, must be implemented as two lines, i.e., one for the positive voltage and one for the negative voltage. Thus, 128 reference voltage lines will be required. For conciseness, only the 64 reference voltage lines for the positive or negative 64 levels will be described in the following description.
The reference voltage level which is supplied to each reference voltage line is basically generated by employing a resistance division technique between a voltage VL obtained from a low voltage reference supply and voltage VH obtained from a high voltage reference supply. For example, the respective 64 reference voltage levels which are supplied to the 64 reference voltage lines can be generated by employing 63 resistors between VL and VH.
FIG. 4 shows a chip layout of a source driver IC 1 functioning as a column electrode driving circuit IC, including a reference voltage generation circuit 7 from which 64 reference voltage lines are provided. The source driver IC 1 (column electrode driving circuit IC) includes: a D/A converter 5; and an output circuit 6. The output circuit 6 is composed essentially of an elongated rectangular IC chip, having 384 data lines in parallel connection provided on one of the longitudinal sides thereof. The 64 reference voltage lines are coupled to the D/A converter 5, which precedes the output circuit 6.
As shown in FIG. 5, the reference voltage lines L1 to L64 are respectively selectable corresponding to gray scale levels 1 to 64 of green. The reference voltage lines L1 to L64 are respectively selectable corresponding to gray scale levels 1 to 64 of red. The reference voltage lines L1 to L64 are respectively selectable corresponding to gray scale levels 1 to 64 of blue. Thus, each of the reference voltage lines L1 to L64 is associated with the same gray scale level (one of 1 to 64), for all of red, green, and blue alike. Accordingly, for each additional gray scale level which may be employed to introduce a greater multitude of gray scale levels in the liquid crystal display panel, there will be an additional reference voltage line required. Note that it is not commonly practiced in the art, when introducing an additional gray scale level, to provide an additional reference voltage line for each color of RGB (which would result in a total of three additional reference voltage lines being employed for RGB) because it is desirable to minimize any significant increase in the number of reference voltage lines, which would occupy a substantial area on the IC chip.
However, the aforementioned structure, in which the same voltage is applied for the same gray scale level irrespective of which color among RGB is addressed, has the following problem. If a given display device of any voltage-driven type has different applied voltage-luminance characteristics (or xe2x80x9capplied voltage-transmittance characteristicsxe2x80x9d in the case of a liquid crystal display) for each of red, green, and blue, then any shift in the luminance of an achromatic display screen from a brighter state to a darker state might result in a varying chromaticity, which would otherwise be constant.
As exemplified in FIG. 6, in the case of a liquid crystal display device, the white-color chromaticity of a display screen tends to shift toward blue as the luminance of the display screen shifts from a brighter state to a darker state. This phenomenon can be explained by the liquid crystal display device possessing different gray scale level-luminance characteristics for red, green, and blue.
FIG. 7 shows an example of gray scale level-luminance characteristics. The axis of abscissas represents gray scale level 1 (dark) to level 64 (bright) of the 6-bit data for each color of RGB. The axis of ordinates represents the luminance of each color of RGB. In the graph of FIG. 7, the luminance level is normalized (unit: %) based on the assumption that any level 64 (i.e., maximum luminosity) 6-bit input data for each color of RGB has a luminance of 100%. From FIG. 7, it can be seen that the luminance value corresponding to the 6-bit data level for each color of RGB is not equal. In order to improve the color displaying performance of image display devices, it is imperative to ensure consistency among the luminance values for the respective colors of RGB.
It might appear that, in order to improve the color displaying performance of image display devices, a set of reference voltage lines (and thus reference voltage levels) could be separately provided in the column electrode driving circuit for each color of RGB, and bit correction could be performed for the 6-bit data. However, such a structure, where every three reference voltage lines would be provided for each color of RGB, would contain 192 (as opposed to 64) reference voltage lines. Since the area of such an IC chip area is inevitably increased, it may become difficult to produce the column electrode driving circuit at low cost.
Alternatively, a method might be possible which involves performing an appropriate calculation for shifting given data, by software means, to a higher or lower value. However, such a method fails to provide a fundamental solution because it would introduce some deterioration in the overall color reproducibility of RGB colors.
According to the present invention, there is provided a column electrode driving circuit for an image display device for selecting, from among a plurality of reference voltage levels, reference voltage levels respectively corresponding to gray scale levels in input data, and outputting the respective selected voltage levels to at least one data line, wherein the input data comprises data of a first color, a second color, and a third color, wherein: the reference voltage levels are independently selected corresponding to the gray scale levels in the input data of the first color, the second color, and the third color; among the reference voltage levels independently selected corresponding to a given gray scale level in the input data of the first color, the second color, and the third color, the reference voltage level selected corresponding to at least one color is different from the reference voltage level or levels selected corresponding to the other color or colors, the given gray scale level being within a predetermined range; and the reference voltage level selected corresponding to the given gray scale level in the input data of the at least one color is equal to a reference voltage level selected corresponding to another gray scale level in the input data of the other color or colors.
In one embodiment of the invention, among the reference voltage levels independently selected corresponding to a given gray scale level in the input data of the first color, the second color, and the third color, the reference voltage levels selected corresponding to the first color and the third color are shifted relative to the reference voltage level selected corresponding to the second color by a predetermined number of gray scale levels; and the column electrode driving circuit provides a number of additional reference voltage levels for interpolation purposes, the number being equal to the predetermined number.
In another embodiment of the invention, luminance values corresponding to gray scale levels of each of the first color, the second color, and the third color when displayed alone are normalized by a first max, a second max, and a third max, which respectively represent the maximum luminance values of the first color, the second color, and the third color when displayed alone; and
the reference voltage levels selected corresponding to the gray scale level are selected so that the luminance values for the first color, the second color, and the third color will coincide on the basis of gray scale level-luminance characteristics expressed in terms of the normalized luminance values.
In still another embodiment of the invention, the first color, the second color, and the third color are red, green, and blue, respectively.
In still another embodiment of the invention, the first color, the second color, and the third color are cyan, magenta, and yellow, respectively.
According to another aspect of the present invention, there is provided an image display device comprising any one of the aforementioned column electrode driving circuits.
In accordance with a column electrode driving circuit for use with an image display device according to the present invention, in areas along the gray scale except for white and black, different reference voltage levels can be applied for a given gray scale level, such that a different reference voltage level can be selected for each of a first color, a second color, and a third color, whereby the luminance values of the first color, the second color, and the third color can be equalized.
Moreover, reference voltage levels corresponding to gray scale levels of the first color and the second color can be offset relative to that of the third color. In that case, additional reference voltage levels for interpolating any two or more values which have become farther apart as a result of offsetting the reference voltage levels can be employed, whereby a constant gray scale resolution can be obtained between such two or more values.
Furthermore, the luminance values corresponding to the gray scale levels of each of the first color, the second color, and the third color when displayed alone may be normalized by a first max, a second max, and a third max, which respectively represent the maximum luminance values of the respective colors when displayed alone, and the reference voltage levels according to the present invention may be set so that the luminance values for the respective colors will coincide on the basis of the gray scale level-luminance characteristics expressed in terms of such normalized luminance values representing gray scale levels. As a result, the unwanted variation in chromaticity associated with changes in the gray scale levels can be minimized.
Thus, the invention described herein makes possible the advantages of (1) providing a column electrode driving circuit with which bit correction for equalizing the luminance values of a first color, a second color, and a third color can be performed without degrading the overall color reproducibility of a color system composed of the first color, the second color, and the third color, with a minimum increase in the IC chip area; and (2) providing an image display device incorporating such a column electrode driving circuit.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.